But if Mail starts choosing "firstname.lastname@example.org" instead of "email@example.com" when you're emailing the boss, you have other options besides going into Contacts, and deleting the rarely-used email address.
To choose a contact's default email address in Mail, you have to:
1. Go to Contacts.
2. Click on All Contacts, then select all. (Go to the Edit menu, and choose "Select All.)
3. Click the File menu, and choose New Group From Selection. Name it something simple like "All".
4. Click the Edit menu, and choose Distribution List.
5. The Distribution List window appears. Click the "All" group you created. Default email addresses will be listed in bold. If you see contacts with multiple addresses, check the bold email address is the one you use most often. If it isn't, just click the one you want to make default.
Extra credit: this is also an easy way to view a simple list of your contacts' names & email addresses. Give it a once-over. You'll be surprised which contacts have no email account at all, or one that is severely out of date. Even more credit: to do the same thing, but viewing/changing phone numbers or mailing addresses, click the "Email" column header while viewing the Distribution List.